Marine power distribution and propulsion systems are well known. In a typical arrangement a series of power converters are used to interface a main ac busbar to a series of loads which can be electric motors, e.g., propulsion motors or thrusters. Other loads can be connected directly to the main ac busbar or connected to an auxiliary ac busbar which is in turn connected to the main ac busbar by means of a transformer. The ac busbars typically operate at different voltages, e.g., 690 VAC and 440 VAC.
Each power converter can be an ‘active front end’ (AFE) converter with a supply-side active rectifier/inverter (or ‘front end’ bridge) having ac terminals connected to the main ac busbar and a load-side active rectifier/inverter connected to the load. The dc output of the supply-side active rectifier/inverter is connected to the dc input of the load-side active rectifier/inverter by a dc link. In normal operation, the supply-side active rectifier/inverter will operate as an active rectifier to supply power to the dc link and the load-side active rectifier/inverter will operate as an inverter to supply power to the load. Each active rectifier/inverter will typically have a conventional topology.
A prime mover (e.g., a diesel engine or turbine) is connected to individual generators which supply power to the main ac busbar. The main ac busbar can be equipped with protective switchgear with circuit breakers and associated controls.
The marine propulsion system will typically include a first (or port) ac busbar and a second (or starboard) ac busbar that are interconnected by a busbar tie. Some marine propulsion systems use a plurality of ac busbar sections or groups interconnected by a plurality of busbar ties to improve power availability.
In one arrangement, shown in FIG. 1, the marine propulsion system 1 includes a power take-in/power take-out (PTI/PTO) hybrid drive system 2. The hybrid drive system 2 includes an induction (or asynchronous) electrical machine 4 and a diesel engine 6. The rotor of the electrical machine 4 and the driving end of the diesel engine 6 are mechanically coupled through a gearbox 8 and are used to drive a propulsion thruster 10, for example. The electrical machine 4 is connected to the main ac busbar 12 by means of an AFE converter 14 with a supply-side active rectifier/inverter 16 having ac terminals connected to the main ac busbar and a machine-side active rectifier/inverter 18 connected to the electrical machine. The dc output of the supply-side active rectifier/inverter 16 is connected to the dc input of the machine-side active rectifier/inverter 18 by a dc link 20. During a PTI mode, the electrical machine 4 is operated as a motor and is used to drive the propulsion thruster. Power is supplied to the electrical machine 4 from the main ac busbar 12 through the AFE converter 14 with the ac supply-side active rectifier/inverter 16 being operated as an active rectifier and the machine-side active rectifier/inverter 18 being operated as an inverter. During a PTO mode, the electrical machine 4 is operated as a generator with the rotor of the electrical machine being driven by the diesel engine 6. Power is supplied from the electrical machine 4 to the main ac busbar 12 through the AFE converter 14 with the machine-side active rectifier/inverter 18 being operated as an active rectifier and the supply-side active rectifier/inverter 16 being operated as an inverter. Diesel generators 22, 24 supply power to the main ac busbar 12.
If the marine propulsion system 1 is in an electric power blackout situation, the diesel generators 22, 24 are non-operational and the electrical machine 4 typically needs to recover the main ac busbar 12 by supplying power to the main ac busbar through the AFE converter 14. In other words, the electrical machine 4 will be driven by the diesel engine 6 and operated as a generator. Embodiments of the present invention provides a way of supplying power to magnetise the electrical machine 4 during a restart process so that it can subsequently supply power to the main ac busbar 12 for recovery purposes during an electric power blackout situation.